1. Field of the Invention
The present invention relates to a servo routine of a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
Information is typically stored in radial tracks that extend across the surface of each disk. Each track is typically divided into a number of segments or sectors. The voice coil motor and actuator arm can move the heads to different tracks of the disks.
FIG. 1 shows a typical track that has a number of fields associated with each sector. A sector may include an automatic gain control (“AGC”) field 1 that is used to adjust the strength of the read signal, a sync field 2 to establish a timing reference for the circuits of the drive, and ID 3 and Gray Code 4 fields to provide sector and track identification.
Each sector may have also a servo field 5 located adjacent to a data field 6. The servo field 5 contains a plurality of servo bits A, B, C and D that are read and utilized in a servo routine to position the head 7 relative to the track. By way of example, the servo routine may utilize the algorithm of ((A−B)−(C−D)) to create a position error signal (“PES”). The PES is used to create a drive signal for the voice coil motor to position the head on the track.
Some disk drives contain a micro-actuator that can provide micro-control of the head position. For hard drives with a micro-actuator the servo will have three loops. A loop for the voice coil motor, a loop for the micro-actuator and an overall loop for the system.
Each servo loop has a corresponding transfer function. For example, the voice coil motor loop has a transfer function and the micro-actuator has a transfer function. Each transfer function can typically be defined by the following equation:ym=A[λ cos((2π/N)k)+β sin((2π/N)k)]  (1)where λ and β are coefficients that contain the magnitude and phase of the transfer function.
The transfer function of a micro-actuator may vary from part to part. Additionally, the transfer function may vary with temperature and time. It is therefore desirable to determine the transfer function of the micro-actuator.
One way to obtain the transfer function of the micro-actuator is determine the transfer function of the voice coil motor and the overall transfer function of the servo system and then calculate the micro-actuator transfer function using the overall and voice coil motor transfer function. This approach can lead to inaccurate results. It would be desirable to more accurately determine the transfer function of a micro-actuator.